Web servers today are a real mixed bag as prices for services have come down so much. To run a successful server swarm needs some resources.
Blade servers are the most common hardware available. Most individual machines can be equipped with at least 128GB of memory. With dual sockets and modern 8 core 16 core thread processors a web server still has problems with heavy workloads. Blades are grouped into a 3U or 4U box and these can be stacked up in EIA racks. A 4U storage box can be added to the rack to provide staggering amounts of storage for web sites.
Apache can use RAM to speed up web pages. Linux also uses RAM to buffer storage. MySQL also uses memory for a buffer. 128GB RAM or more can be leveraged.
Testng our server in Azure servers, Linux was able to use even the top server plan without problems. Top showed very little free memory showing that memory is important for web servers.
WordPress is 22MB and with some themes and plugins it maybe 50MB. So a 4U 10PB storage box can host staggering numbers of web sites. Apache runs one transitory instance for each HTTP GET. So it makes sense to run Linux from a fast SSD system disk. The new M.2 SSD products have more bandwidth which can make servers runs much better.
4U storage servers are so inexpensive that it makes sense to have several of them. The idea is to have multiple layers of backups for fault tolerance. rsync can be run a as cron task as needed to mirror user folders. The usual disclaimers should be posted in case of the unthinkable.